The present invention relates to a solid state imaging device of a digital still camera, a digital video camera, and the like.
Conventionally, a solid state imaging element which converts received light to an electric signal and outputs the electric signal as a video signal has been known, and a camera which displays the video signal obtained from the solid state imaging element in the form of a static image, such as a digital still camera, or the like, has also been known. In recent years, further improvements of image quality and functions have been demanded in such a camera which uses a solid state imaging element, and the number of pixels has been rapidly increasing.
For example, a solid state imaging element having about 5,000,000 pixels has about 1,920 pixels in a column (vertical direction) and about 2,560 pixels in a row (horizontal direction). The number of pixels of this element, i.e., about 5,000,000, is about 16 times that of a generally-employed NTSC solid state imaging element. The frame rate for full pixel output is about a ½ second when a conventional pixel clock of about 12 MHz is used. Thus, the video signal output from the solid state imaging element cannot be output to a display device (a liquid crystal monitor, or the like) of the camera without modifying the original frame rate.
According to a driving method which has been conventionally employed in view of the above in such a solid state imaging element, the pixels from which signals are to be read are thinned along the horizontal direction while the speed of the pixel clock is increased, whereby a video signal of a moving picture is read with higher speed. For example, signals of pixels on two out of eight lines are used.
Further, a technique of reducing the number of output pixels of a solid state imaging element using a pixel mixture method has been known (see Japanese Unexamined Patent Publication No. 2001-36920).
However, in the above pixel thinning method, pixels are immoderately resampled in the vertical direction (¼ in the above example), and no associated spatial LPF used for this resampling in the vertical direction is not provided. Accordingly, in an image where a video signal contains high-frequency components in the vertical direction, a large amount of aliasing components deriving from the high-frequency components in the vertical direction occur in the low-frequency range. This causes not only a large number of false signals along with the generation of luminance signals and chromaticity signals but also a significant decrease in the vertical resolution with respect to the horizontal resolution due to an imbalance in pixel sampling density between the horizontal direction and the vertical direction. In addition, since signals of pixels on lines from which no data is to be read out are discarded, the substantial sensitivity decreases. In the above example, the percentage of effectively-used pixels is 25%.
In the case where the above-described conventional technique is used, all of the above problems become more serious in principle as the number of pixels of a solid state imaging element increases because it is necessary to decrease the ratio of columns to be read to all of the columns of the solid state imaging element for the purpose of increasing the frame rate.